Suction device for a supercharged engine

ABSTRACT

In constructing a suction device for a crank chamber supercharged engine 1 wherein intake air pressurized by a supercharge mechanism is supplied to a combustion chamber, it is arranged that the supercharge mechanism and the combustion chamber are communicatively interconnected by suction passage 38, 46, with an intake valve 7 disposed open/close operably at a downstream end of the suction passage 46, the intake valve 7 being adapted to close between 70 and 100 degrees after a bottom dead center.

TECHNICAL FIELD

The present invention relates to a suction device for a supercharged4-cycle engine having a supercharge mechanism.

BACKGROUND ART

There has been proposed a supercharged 4-cycle engine, for example, suchthat a connecting rod housing chamber surrounded by a crank chamber, acrank web, and a piston is partitioned by a connecting rod into asuction chamber and a compression chamber so that the air sucked throughthe rocking of the connecting rod is compressed to be supercharged intothe combustion chamber (see Japanese Patent Application Laid-Open No.6,93869).

A suction device for such a supercharged engine has been known such thata chamber having a comparatively large capacity is communicativelyconnected to the compression chamber, the chamber and the combustionchamber being communicatively interconnected by a suction passage, witha carburetor disposed in the suction passage. This chamber is to storepressurized air therein thereby to inhibit an air pressure drop during asuction stroke so as to enhance the efficiency of charging.Conventionally, the capacity of the chamber has been set as large aspossible, even on the order of, for example, more than 20 times thequantity of engine displacement.

However, when such a supercharged engine is employed in, for example, asmall-size vehicle, such as automotive two-wheel vehicle, one problem isthat it is difficult to secure necessary space for disposition of such alarge-capacity chamber.

Another problem is that, in the case of a supercharged engine of theabove mentioned type, if the supercharged pressure is increased when thecompression ratio is set at a usual level, there will occur knockingsuch that the output of the engine does not go up even if the quantityof the mixture is increased.

This invention has been made in view of the foregoing situation of theart and has as its object the provision of a suction device for asupercharged engine which can prevent knocking when the engine isequipped with a supercharge mechanism and can realize a size reductionto enable use in a small type vehicle.

DISCLOSURE OF THE INVENTION

The invention of claim 1 presents a suction device for a superchargedengine wherein intake air pressurized by a supercharge mechanism issupplied to a combustion chamber, characterized in that where thecapacity of a suction passage communicatively interconnecting thesupercharge mechanism and the combustion chamber is V and displacementis V0, V/V0 is set within the range of from 1 to 20 and in that anintake valve for opening and closing a combustion chamber opening at adownstream end of the suction passage is closed within the range of from70 to 100 degrees after a bottom dead center.

When the "displacement" of the combustion chamber (V0) is referred to inthis specification and the claims, the term "displacement" is used inthe normal sense applied in referring to engine capacity or size. Thatis, the displacement of a single cylinder is the volume displaced by thepiston during a complete stroke of operation. Thus, if a cylindricalbore is employed, the displacement is equal to πD÷4×S where D equals thediameter of the cylinder bore and S equals the stroke of the piston.

The invention of claim 2 is characterized in that in the invention ofclaim 1 the cross-sectional area of the suction passage is generallyuniform over the entire length of the passage.

In the invention of claim 2, the expression "the cross-sectional area isgenerally uniform" means that the passage has no part which positivelyfunctions as a storage chamber for pressurized air. The time for closingthe intake valve may be fixed within the above angular range, or may bevariable within and outside the angular range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view for explaining a suction device for asupercharged 4-cycle engine which represents one embodiment of theinvention;

FIG. 2 is a sectional rear view (a section taken on line II--II inFIG. 1) showing a crank shaft portion of the engine representing theabove embodiment;

FIG. 3 is a sectional side view of a carburetor in the above embodiment;

FIG. 4 is a sectional rear view of the carburetor of the aboveembodiment;

FIG. 5 is a side view of a scooter type automotive 2-wheel vehicle inwhich the engine of aforesaid embodiment is employed; and

FIG. 6 is a time chart showing times for valve operations in the engineof the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode in which the present invention is carried out will now bedescribed with reference to the accompanying drawings.

FIGS. 1 through 6 are views for explaining a suction device for a4-cycle supercharged engine according to one embodiment of the presentinvention, in which FIG. 1 is a sectional side view of the enginerepresenting the embodiment; FIG. 2 is a section taken on line II--II inFIG. 1; FIGS. 3 and 4 are a sectional side view and a sectional rearview respectively of a carburetor portion; FIG. 5 is a side view of ascooter type automotive two-wheel vehicle carrying the engine of theembodiment; and FIG. 6 is a time chart for valve operation times. In thepresent embodiment, the words "left" and "right" mean left and right asviewed in the direction of forward movement of the vehicle.

In the drawings, reference numeral 200 designates a scooter typeautomotive 2-wheel vehicle which includes a body frame 201 of theunderbone type comprising a main frame 201b having an L-shapedconfiguration as viewed in side elevation, the upper end of which isconnected to a head pipe 201a, a pair of side frames 201c, 201c, rightand left, connected at front ends thereof to the lower end of the mainframe 201b, thereby the side frames 201c being slanted up, the rear endsof the side frames being extended rearward. A front fork 203 at thelower end of which a front wheel 202 is rotatably supported is laterallycontrollably supported on the head pipe 201a. A steering handlebar 204is fixed to the upper end of the front fork 203.

The steering handlebar 204 is covered around it with a handle cover 205,and the head pipe 201a is covered around it with a front cover 206 of2-split construction consisting of a front cover 206a and a rear cover206b. A foot board 207 which constitutes a foot mount is provided atjunction between the main frame 201b and the side frames 201c, and sidecovers 208, right and left, are provided alongside the side frames 201c.A sheet 209 is placed on the side covers 208 each, and below the sheet209 there are mounted a store box 210 and a fuel tank 49 in position.

A unit swing type engine unit 214 is mounted adjacent a location belowthe sloped-up portions of the right and left side frames 201c. Thisengine unit 214 comprises an engine body 1 to be hereinafter describedwhich is vertically pivotally supported in suspension by a pivot shaft213 mounted between the side frames 201c through the intermediary of asuspension bracket 212, and a rear wheel transmission gear 215 formedintegrally with and in continuation to the engine body 1 at the leftside thereof for rearward extension. The transmission gear 215incorporates a belt-drive type nonstep variable speed gear (not shown)therein, and at a rear end portion of the gear a rear wheel 216 isrotatably supported. A rear cushion 217 is rotatably supported at itslower end on the top of a rear end portion of a transmission case 215aof the rear wheel transmission gear 215, the upper end of the rearcushion 217 being rotatably supported by the side frame 201c.

An air cleaner 218 is disposed in a corner portion defined by a leftside wall of the engine body 1 and a front wall of the transmission case215a, the air cleaner 218 being securely bolt-fastened to both theengine body 1 and the transmission case 215a. The air cleaner 218 ispivotally movable in association with the engine unit 214. Therefore, norelative movement occurs between the two 218 and 214. This eliminatesthe need for flexibility in communicatively connecting the two and canprovide for simplicity in construction. The air cleaner 218 may bedisposed on the top of the transmission case 215a or on the top of theengine body 1. An oil tank 220 is mounted at right side of the enginebody 1 and a carburetor 100 on the top of the engine body 1. Further, anexhaust pipe 221 is connected to the underside of the engine body 1,the-rear end of the exhaust pipe 221 being connected to a muffler 222disposed on the right side of the rear wheel 216.

Next, the engine body 1 in the present form of application will beexplained in detail.

The engine body 1 is a 4-cycle single cylinder supercharged cylinderwith a crank shaft laterally disposed. An air cooled cylinder block 3 isconnected to a front mating surface of the crankcase 2 of the engine 1,and a cylinder head 4 is connected to a front mating surface of thecylinder block 3. A portion of the front mating surface of the cylinderhead 4 which is opposed to a cylinder bore 3a is recessed to form acombustion recess 4a which constitutes the combustion chamber, with anignition plug not shown being inserted in the recess 4a.

An intake port 5 and an exhaust port 6 are open in the combustion recess4a of the cylinder head 4, the intake port 5 and exhaust port 6 beingdrawn to an upper wall side and a lower wall side respectively of thecylinder head 4. An intake valve 7 is open/close operably disposed inthe combustion chamber opening of the intake port 5, and an exhaustvalve 8 is open/close operably disposed in the combustion chamberopening of the exhaust port 6. The intake valve 7 and the exhaust valve8 are biased by a valve spring 9 in the direction of closing movement.

In the cylinder head 4, a cam shaft 10 which acts as an actuationmechanism for open/close actuating the valves 7, 8 are disposed in adirection perpendicular to the surface of the drawing sheet. One end ofthe cam shaft 10 is connected to a sprocket 13 (see FIG. 2) joined witha crank shaft 20 to be described hereinafter, through a sprocket and achain not shown, and the cam shaft 10 is decelerated to one half of therotational speed of the crank shaft 20. On the upper and lower sides ofthe cam shaft 10 there are disposed a pair of rocker shafts 11, 11extending in parallel therewith. Respective rocker shaft 11 has a rockerarm 12 pivotally mounted thereto. One end of the rocker arm 12 abuts acam nose of the cam shaft 10, and the other end abuts the upper ends ofthe intake and exhaust valves 7, 8.

The configuration of the intake cam nose is designed so that the intakevalve 7 is opened 10 to 15 degrees prior to the top dead center ofpiston 16 and is closed within the range of 70 to 100 degrees after thebottom dead center. The configuration of the exhaust cam nose isdesigned so that the exhaust valve 8 is opened 40 to 60 degrees prior tothe bottom dead center and is closed 10 to 20 degrees after the top deadcenter. The timing for ignition is set at 5 to 25 degrees before the topdead center.

The piston 16 is slidably disposed in a cylinder bore 3a of the cylinderblock 3. With this piston 16 as a boundary, a crank chamber 23 isdefined by the combustion chamber, the cylinder bore 3a oppositethereto, and the crank case 2. A smaller end 17a of the connecting rod17 is connected to the piston 16 through a piston pin 18 and a bearing19, and a larger end 17b of the connecting rod 17 is connected to acrank pin 21 of the crank shaft 20 through a bearing 22.

The crank shaft 20 is housed in a crank case compartment 23a of thecrank chamber 23 and includes a pair of crank webs 24 connected togetherby the crank pin 21, and a journal 25 integrally formed with each crankweb 24, the journal 25 being supported by the crank case 2 through ajournal bearing 26. The journal 25 projects outwardly from the crankcase 2, and has another projection on which a generator 28 is mounted.Shown by 27 is an oil seal.

Inner side walls 2a, right and left, of the crank case 2 which cross thecrank shaft 20 of the crank case 2 at right angles are flush withnotches 3c formed, as shown, above and below the engagement portion 3bof the cylinder block 3 relative to the crank case 2. Sides 17c, rightand left, of the connecting rod 17 are in slide contact with these innerside walls 2a, right and left, and notches 3c with a minute clearance,and the outer periphery of the larger end portion 17b of the connectingrod 17 are in slide contact with the arcuate inner periphery 2b of thecrank case 2 which is so formed as to surround the crank shaft 20, suchslide contact being held with a very minute clearance. The inner sidewalls 2a, right and left, of the crank case 2 are recessed to formcircular recesses 2c in which the crank webs 24 are fitted for placementtherein, with a very slight clearance provided between respective recess2a and corresponding crank web 24. Also, on the connecting rod side,side walls 24a are in slide contact with the sides 17c of the connectingrod 17 with a minute clearance. At each respective mouth of the rightand left circular recesses 2c, a ring 2d is disposed as fixed to thecrank case 2 so that the ring 2d is in contact with or in slide contactwith the projecting outer periphery of the crank web 24 but withsubstantially zero clearance.

Aforesaid rings 2d are made of a material having a higher degree ofhardness or higher resistance to wear as compared with the crank case.

A skirt portion of the piston 16 is formed with a triangular recess 16a,and on the outer periphery of the skirt there is formed a notch 16bopposite to the recess 16a. The smaller end portion 17a of theconnecting rod 17 is fitted in the recess 16a. The inner periphery ofthe recess 16a is held in slide contact with the outer periphery of thesmaller end portion 17a, and the sides 16c, right and left, of therecess 16a are held in slide contact with the sides 17c, right and left,such slide contact being held with a minute clearance.

Through the above described arrangement, a portion which is surroundedby the crank chamber 23, crank web 24, and piston 16 constitutes aconnecting rod housing chamber. In angular crank positions except wherethe piston 16 is positioned adjacent the top dead center, the connectingrod 17 fits in the space between the inner side walls 2a, right andleft, of at least one of the crank cases 2, upper and lower, or in thenotch 3c of the cylinder block 3, the connecting rod housing chamberbeing partitioned into a suction chamber A and a compression chamber Bby the connecting rod 17. When the piston 16 is positioned at the topdead center, there is no fitting engagement between the connecting rod17 and the inner side walls 2a, 2a, right and left, but the skirt end ofthe piston 16 substantially coincides with the end of the cylinder bore3a, so that the division between the suction chamber A and thecompression chamber B as defined by the piston rod 17 is maintained. Inthis way, as the crank shaft 20 rotates clockwise from the state inwhich the piston 16 is positioned at the top dead center, as FIG. 1shows, the connecting rod 17 moves to the chain line position, then tothe chain double-dashed line position, and then to the solid lineposition, so that the capacity of the suction chamber A is increased toallow air suction and the capacity of the compression chamber isdecreased so that the air taken in during the previous stroke iscompressed. Thus, a capacity-type supercharger configuration is formed.Such a construction is disclosed in detail in the above cited JapanesePatent Application Laid-Open No. 6-93869.

On the underside of the crank case 2 there is formed integrallytherewith a suction passage 35 communicating with the suction chamber Awhich is open downward. On the foreground-side wall surface of thesuction passage 25 in FIG. 1 there is formed an intake port 35a inmanner as oriented toward the crank shaft. The intake port 35a isconnected to the downstream end of suction pipe 37. The suction pipe 37passes along a side of the engine and extends outward. The upstream endof the suction pipe is connected to the air cleaner 218.

On the top of the crank case 2 there is integrally formed therewith asuction passage 38 communicating with the compression chamber B andopening upward, and in the suction passage 38 there is integrally formedtherewith a partition wall 39 for separating the passage 38 from thecompression chamber B. The partition wall 39 is formed with a valveopening 140 for communicating the suction passage 38 with thecompression chamber B. The open area of the valve opening 140 is set tosuch a size as to enable sufficient reduction of air pressure.

A reed valve 42 for opening and closing the valve opening 140 isdisposed on the exterior of the partition wall 39, The reed valve 42comprises a valve plate 41a made of a light metal material composedmainly of titanium, and a stopper 41b disposed on the lift side andoperative to regulate the valve travel of the valve plate 41a. Thestopper 41b and valve plate 41a are fixedly bolted together by bolt 43at one end.

The upstream end of the suction pipe 46 is connected to the suctionpassage 38, and the downstream end of the suction pipe 46 is connectedto the intake port 5 of the cylinder head 4. The suction passage 38 andthe suction pipe 37 are in communication with each other through abypass passage 50. The bypass passage 50 is freely open/close operableby a bypass valve 51. The bypass valve 51 is of such a construction thata valve body 51a is moved to an opening position by the negativepressure within a diaphragm chamber 52. The diaphragm chamber 52 is heldin communication with the downstream end of the suction pipe 46 througha negative pressure passage 53. The bypass valve 51 is such that whenintake negative pressure is increased in the case of a small throttletravel, for example, the bypass passage is opened by the negativepressure to reduce the pressure within the suction passage 38.

A carburetor 100 is disposed at a midway location on the suction pipe46. The carburetor 100 is a float chamber 102 operable as a fuelreservoir which is bolt fastened to an underside opening of a cab body101. The cab body 101 comprises a cylindrical body portion 101a having aventuri passage 103 in which intake air flows, a cylinder portion 101bextending vertically upward from the body portion 101a and open at aventuri portion 103a of the venturi passage 103, and a fuel inductionportion 110c extending into the float chamber 102 coaxially with thecylinder portion 101b, all these component members being integrallyformed together.

A lid 104 is threadingly attached to a top end opening of the cylinderportion 101b, and a cylindrical variable piston valve 105 with a bottomis axially slidably fitted in the cylinder portion 101b. Between thepiston valve 105 and the lid 104 there is disposed a spring 106 fornormally biasing the piston 105 in the direction of valve closing. Athrottle cable 107 is connected at one end to the piston valve 105, andan extension end of the cable 107 is connected to a throttle grip (notshown) of aforesaid steering handlebar 204 after passing through a guidetube 108 connected to the lid 104. Through the control of the throttlegrip the piston valve 105 is caused to move vertically to vary thepassage area of the venturi portion 103a between full closing and fullopen. The cylinder portion 110b is fitted with a detent screw 109 whichengages an elongate groove provided in the piston valve 105 and acts asa whirl stop for the piston valve 105.

An air bleed sleeve 110 is fitted in the fuel induction portion 10c, anda fuel discharge port 110a of the sleeve 110 is open beneath the piston105 in the venturi portion 103a. A main jet 111 is threadingly mountedto the lower end of the air bleed sleeve 110 and is open within thefloat chamber 102. A needle 112 is back and forth movably inserted inthe air bleed sleeve 110, the upper end of the needle 112 extendingthrough the bottom of the piston valve 105 and fixed thereto. As thepiston valve 105 moves upward and downward, the needle 112 moves backand forth to vary the effective area of the fuel discharge port 110athereby to adjust the quantity of fuel sucked into the venturi passage103.

Downstream of the fuel induction portion 101c there is formed a slowfuel induction portion 101e in which a slow jet 115a is disposed byinsertion. Upstream of the fuel induction portion 101c there is formed ableed air inlet hole 101f which is open toward the suction pipe 46, withan air jet 116 disposed therein through insertion. A side wall of theair bleed sleeve 110 is formed with an air bleed hole not shown.

The cab body 101 is formed with a fuel inlet port 101d whichcommunicates with the interior of the float chamber 102. The inlet port101d is communicatively connected to one end of a fuel supply hose 113,the other end of the supply hose 113 being communicatively connected tothe bottom of a fuel tank 49. The fuel tank 49 is positioned above thecarburetor 100 so that fuel is supplied into the float chamber 102 bygravity. A pressure pipe 114a communicatively connected to the suctionpassage 38 is connected to the ceiling wall of the fuel tank 49 so thata supercharge pressure is applied to the fuel in the fuel tank 49. On atank cap 114b of the fuel tank 49 there is disposed a relief valve 114cwhich is adapted to open when a supercharge pressure greater than apredetermined pressure is applied.

Disposed within the float chamber 102 is a float 120 which is verticallypivotally supported by a plate 118 and a support shaft 119. A fuelintake valve 117 is disposed at the fuel inlet port 101d in the floatchamber 102, and the lower end of a valve body 117a of the intake valve117 is held in abutment with and supported by the plate 118. Thus, withchanges in the fuel level in the float chamber 102, the float 120 movesupward and downward so that the fuel inlet hole is opened and closed bythe valve body 117a whereby the fuel level in the float chamber 102 ismaintained constant.

In the cylinder portion 101b of the cab body 101 there is axially formeda pressure passage 125 which extends in the axial direction, the lowerend of the pressure passage being open within the float chamber 102. Oneend of a pressure hose 126 is connected to the upper end of the pressurepassage, the other end of the pressure hose 126 is communicativelyconnected to the interior of the suction passage 38. Thus, thesupercharge pressure within the suction passage 38 is applied to theliquid level within the float chamber 102.

In this way, the cab body 101 is provided with a pressure passage 125which communicates with the float chamber 102, and the pressure passage125 and the communication passage 38 are kept in communication by thepressure hose 126 with each other, Therefore, even if the pressure inthe venturi portion 103a becomes high, a pressure larger than thepressure at the fuel discharge port 110a of the venturi portion 103a canbe applied to the fuel surface in the float chamber 102, whereby fuelsupply can be stably made. In this case, even if there is any change inthe pressure at the discharge port 110a, a pressure corresponding tosuch pressure change acts on the interior of the float chamber 102. Thispermits sufficient fuel supply which in turn results in improved engineoutput and more effective fuel consumption.

The fuel in the fuel tank 49 is supplied by gravity. This, coupled withthe fact that supercharge pressure is applied to the interior of thefuel tank, makes it possible that in an operating zone where thesupercharge pressure is low, fuel is supplied from the fuel tank intothe float chamber 102 in the same manner as in the case of aconventional carburetor type fuel supply device. In the superchargedoperation zone, supercharge pressure is applied on both the fuel tank 4and the float chamber 102. Therefore, good fuel pressure balance can beobtained and, as stated above, fuel is supplied by gravity from the fueltank 49 to the float chamber 102. In this way, fuel can be supplied tothe supercharged engine 1 without use of a fuel pump and/or a specialcarburetor. This results in cost reduction and mounting space reduction.

Further, the supercharge pressure within the suction passage 38 isapplied to the interior of the float chamber 102. This makes it possibleto reduce pressure variations and to perform fuel feed in a more stablemanner. It is not necessary to constantly pressurize fuel by means of afuel jet device or the like. Therefore, problems such as time lag infuel pressure pull-up at the time of starting, and increased powerconsumption can be solved. Further, in the event of battery discharge,fuel supply can be performed, which serves as effective safety measures.

Numeral 60 designates a lubricating device which comprises a firstlubricating system 61 and a second lubricating system 62. The firstlubricating system comprises a first reservoir tank 631 loaded with a4-cycle purpose oil, and an oil supply pipe 65 connected to thereservoir tank, with a supply pump 64 interposed between them, a supplyport 65a of the supply pipe 65 being connected to the cam shaft 10 whichconstitutes a movable valve mechanism of the cylinder head 4. Oil usedin lubricating the cam shaft 10 returns to a chain chamber 10b in whichabove mentioned chain is housed, wherein the oil lubricates drivensprockets and the left side journal bearing 26 (with sealing) in FIG. 2.After lubrication, the oil is collected into the reservoir tank 63 afterpassing through a collecting pipe 66.

The second lubricating system 62 comprises a second reservoir tank 70loaded with a 2-cycle purpose oil, and a main supply pipe 71 and apressure feed pump 72 which are connected to the reservoir tank, andfirst and second auxiliary supply pipes 73, 74 connected to the pump 72,the first auxiliary supply pipe 73 being connected to the piston slideportion of the cylinder block 3, the second auxiliary pipe 74 beingconnected to the journal bearing portion of the crank chamber 23.

The pressure feed pump 72, not shown though, is an improvement of anelectromagnetic pump which comprises an improved solenoid, the feed pumpcomprising an armature securely fixed to a push rod of a plunger suchthat the armature is attracted by the solenoid. Through this arrangementthe discharge pressure of the pump 72 is increased so that the pressureovercomes the supercharge pressure to enable oil feed.

The first auxiliary supply pipe 73 extends through a liner portion ofthe cylinder block 3 in a direction rectangular to the crank shaft 29,and is positioned more adjacent to the crank chamber than the secondpiston ring of the piston 16 which is located at the bottom dead center.On an external surface more close to the crank chamber than the secondpiston ring of the piston 16 there are formed two parallel notches inthe direction of the crank shaft to define two oil reservoir recesses75, 76. The recess 75 is lubricated with oil supplied from the dischargeport 73 when it coincides with discharge port 73a through the movementof the piston 16, the lubrication being made when the discharge port 73ais closed by the piston 16. The recess 76 is lubricated with oilsupplied from the discharge port 73a and accumulated, in manner asdescribed hereinafter, when the discharge port 73a is closed by thepiston 16. In this way, any possible trouble can be avoided which mayoccur in connection with lubrication as a result of the discharge port73a being closed by the piston 16.

On the outer periphery of the smaller end portion 17a of the connectingrod 17 there is formed an oil guide groove 81 extendingcircumferentially of the end portion. The piston 16 is formed with acommunication hole 77 which communicates with the guide groove 81 andthe lower recess 76. On the opposite side of the communication hole 77there is formed a communication hole which communicates with the pistonpin 18 and bearing 19. Through this arrangement, part of the oil in theguide groove 81 which is supplied from the discharge port 73a and theoil collecting hole 80 is supplied from the communication hole 78 to thebearing 19, and part of the remaining oil is supplied from thecommunication hole 77 to the slide contact surface of the smaller endportion 17a and the piston 16, and further to the recess 76.

The oil discharge port 74a of the second auxiliary supply pipe 74extends through the crank case 2 until it reaches a journal bearing 26at the right side as in FIG. 2. The oil supplied to the right journalbearing 26 is supplied to respective inner walls 2a, 2b of the crankcase 2, and the clearance between the circular recess 2c and the crankshaft 20, and respective slide surfaces.

The partitioning wall portion 39, at a lower portion of its inclinedsurface, is formed with a small diameter oil collecting hole 80 whichcommunicates with the suction passage 35. The cover portion 36 attachedto a lower portion of the crank case 2 is formed with a collectingpassage 83. The collecting passage 83 is connected to a collecting pipe84 which in turn is connected to the second reservoir tank 70.

Thus, the oil supplied from the first and second auxiliary supply pipes73, 74 gathers on the upper surface of the cover portion 36 in thesuction passage 35 from which the oil is collected. In the presentembodiment, the engine 1 is a horizontal type such that the compressionchamber having high supercharge pressure is disposed in an upward facingposition and air is sucked from below the crank case 2. Therefore, byinteraction of gravity and supercharge pressure, lubricating oil gathersin a lowermost portion, which assures accurate collection of oil, thusenabling recycling of the oil. In the crank case 2, with the seal ringof the seal ring-attached bearing 26, at the left side in FIG. 2, as theboundary, the crank chamber 23 side is lubricated by 2-cycle oil and thechain chamber 10b side, including ball portion of the seal ring attachedbearing, is lubricated by 4-cycle oil.

Next, the function and effects of the present embodiment will beexplained.

In use of the engine 1, each time the crank shaft rotates one turn, thefollowing quantity of air is pressure-fed from the suction passage 38 tothe suction pipe 46, namely, volume V2 of the compression chamber B atthe moment when the larger end portion of the connecting rod 17approaches the arcuate peripheral wall of the crank case 2 and justbefore it begins sliding contact, less a minimum volume V3 of thecompression chamber B at the moment when the connecting rod approachedmost close to the partitioning wall portion. That is, the balance V1 isthe quantity of air to be pressure-fed from the suction passage 38 tothe suction pipe 46.

The volume V of the suction passage 200 consisting of the intake passage38, suction pipe 46 adjacent the carburetor 100, venturi passage 103 inthe carburetor 100, and suction port 5 is set to be within the range offrom 1 to 20 times the quantity of displacement (bore×stroke) V0 by onestroke of piston 16. Thus, when the suction valve 7 is closed at the endof a previous suction stroke, fresh air of volume 2 V1 from thesupercharge mechanism is forced into the fresh air retained in thesuction passage 200, a space of volume V, and is stored therein. Duringa succeeding suction stroke in which the intake valve 7 is open, the airin the suction passage 200 enters the combustion chamber. That is, thepressure in the suction passage 200 is increased generally to the orderof (2 V1+V)/V. Then, during a suction stroke of the engine 1, themagnetic valve is opened, whereupon two times the quantity of pressureair feed V1 for one turn of the crank shaft 20 is supercharged into thecombustion chamber.

The ratio of V1/V0 is intended to be more than 1, and the quantity offresh air charge due to supercharging is increased (compression ratio isnominally (2V1+VC)/VC, but due to leakage from slide contact portionswith which the connecting rod 17 is in slide contact but withoutcontact, such as arcuate peripheral wall 2b and side walls of crank web24, and also due to a slight degree of back flow at the reed valve,actual value will be smaller than the nominal value) and knocking mayoccur.

In the present engine 1, the intake valve 7 will close when the crankangle reaches 70-100 degrees from the bottom dead center of the piston16. Therefore, the compression ratio can be reduced substantially andhence knocking that may otherwise occur in the pressurizing of intakeair by the supercharge mechanism can be prevented. If compression ratioonly is reduced, there will be no change in the expansion ratiodetermined by displacement (bore×stroke) and combustion chambercapacity. Therefore, any effect upon fuel consumption and engine outputcan be avoided.

Since the timing for closing the intake valve is delayed, there may be afear of blow back of the mixture. However, the capacity V1 of thesuction passage 200 is of such a small order of 1 to 20 times thedisplacement V0, and therefore any such blow back can be inhibited and asubstantially same quantity of air charge can be obtained as in the casewhere the timing for intake valve closing is same as that in the priorart and where the capacity V1 is more than 20 times the displacement V0.

In the present mode of the invention, a large capacity chamber isunnecessary, and therefore it is possible to provide a superchargedengine 1 which is small-sized, compact and light, and which can beemployed in a scooter type automotive two-wheel vehicle 200.

Since the timing for closing the intake valve 7 is delayed, and sincethe air from the supercharge mechanism is directly supplied into thecombustion chamber, the intake air is more liable to cool than in thecase where the air is supplied through a chamber. Therefore, anintercooler for cooling the pressurized air from the superchargemechanism is unnecessary, and in this respect, too, the invention canwell meet the need for size reduction and more compact construction.

Since the capacity of the suction passage 200 is reduced, the responsefunction in feeding the pressurized air from the supercharge mechanismis improved, and therefore pull-up for acceleration can be prompted somuch, resulting in improvement in the acceleration performance. In thecase of deceleration, reduction of supercharge pressure is quicklyperformed, resulting in improved transient characteristics. Furthermore,since a large-capacity chamber is unnecessary, there is no possibleretention of fuel from the carburetor 100 and, therefore, the effect ofsuch retention on the mixture ratio is avoided so that engine stop andpoor starting can be prevented and any possible damage in the event ofback fire can be prevented.

In the foregoing mode of the invention, a 4-cycle, single cylinder crankchamber supercharged engine is described by way of example but, ofcourse, the invention is applicable to engines equipped with asupercharge mechanism of other type than that of the present invention,such as turbocharger or supercharger, and is of course applicable tomulti-cylinder engines. In the above described mode, the invention isapplied to a scooter type automotive 2-wheel vehicle, but the scope ofapplication of the invention is not limited to such a vehicle and is ofcourse applicable to other kinds of vehicles as well.

INDUSTRIAL APPLICABILITY

As described above, in the suction device for supercharged engineaccording to the invention claimed in claim 1, the suction valve isclosed in the angular range of 70 to 100 degrees after the bottom deadcenter and, therefore, only compression ratio can be reduced withoutchanging the expansion ratio as determined by bore×strokes and capacityof the combustion chamber, knocking can be prevented, and waste in fuelconsumption can be prevented.

Where the timing for closing the suction valve is delayed, sucked airflowing into the combustion chamber is liable to blow back into thesuction passage. In the present invention, however, the capacity of thesuction passage is set within the range of 1 to 20 times the quantity ofdisplacement, that is, there is no part which positively functions as apressurized air storage chamber, such flow back can be inhibited, andthe timing for closing the suction valve can be maintained at a levelsimilar to that in the prior art, it being thus possible to secure sucha substantially same quantity of air charge as is available from adevice having a large capacity of suction passage.

Where the need for chambers is eliminated, it is possible to realizereduction in size of the supercharged engines as a whole, and to enableapplication of the invention to small size vehicles.

What is claimed is:
 1. A crankcase supercharged engine comprised of acylinder having a cylinder bore with a crankcase chamber formed at oneend thereof, a piston reciprocating in said cylinder bore, a crankshaftrotatably journalled in said crankcase chamber, a connecting rod coupledto said piston and said crankshaft for transmitting motion therebetween,means for providing a seal between one end of said connecting rod andsaid piston and between the sides of said connecting rod and the sidesurfaces of said crankcase chamber, said connecting rod having a portionthereof in sealing engagement with said crankcase during at least aportion of a single rotation of said crankshaft for dividing saidcrankcase chamber into a pair of variable volume chambers formed by saidpiston, said cylinder bore, said connecting rod, said crankshaft andsaid crankcase chamber for acting as a positive displacementsupercharger mechanism having two pumping cycles per each revolution ofsaid crankshaft, intake means for admitting an air charge to saidcrankcase chamber, and a suction passage for discharging a compressedair charge from said crankcase chamber through an intake valve to acombustion chamber formed at least in part by said piston and saidcylinder bore, wherein the volumetric capacity of said suction passageis V and displacement of said combustion chamber is V0, the ratio V/V0being within the range of from 1 to 20, and said intake valve beingclosed within the range of from 70 to 100 degrees after a bottom deadcenter position of said piston.
 2. A suction device for a superchargedengine as set forth in claim 1, wherein the cross-sectional area of thesuction passage is substantially uniform over the the length of saidsuction passage.